1. Field of the Invention
The present invention relates to a light wavelength conversion module, which converts a fundamental wave to a second harmonic wave or the like by using a light wavelength conversion element, and to a light scanning and recording apparatus using the light wavelength conversion module. More precisely, the present invention relates to a light wavelength conversion module using a light wavelength conversion element formed of a bulk-shaped wavelength conversion crystal, and to a light scanning and recording apparatus utilizing the light wavelength conversion module.
2. Description of the Related Art
Conventionally, various methods for converting a fundamental wavelength to a second harmonic wavelength or the like have been proposed. For example, Bleombergen et al. have proposed a method for converting a fundamental wavelength to a second harmonic wavelength by using a light wavelength conversion element having a region in which a spontaneous polarization (domain) is reversed periodically (Phys. Rev., vol. 127, No. 6, 1918 (1962)).
The wavelength conversion efficiency is proportional to a length interacting with the fundamental wave which is substantially an excitation light, i.e., the crystal length, and to the square of the power density of the excitation light. As the light beam diameter of the excitation light is made smaller in the wavelength conversion crystal in order to obtain a higher conversion efficiency, the power density of the light increases and the conversion efficiency improves accordingly. On the other hand, since a narrow beam diffuses drastically as the beam is propagated, the interactive length in the region where the power density is high is shortened and the conversion efficiency decreases accordingly. That is, there is a trade-off between power density and the interactive length. Due to this relationship, it is known that the conversion efficiency of the wavelength conversion element formed by bulk-shaped wavelength conversion crystal is relatively high when the beam diameter of the fundamental wave is several tens of μm. It is easy to perform optical adjustment of a beam having a diameter of several tens μm by inputting the beam into a wavelength conversion crystal which is, for example, about 1 mm3. Consequently, an inexpensive light source with a small number of component parts can be used. Therefore, the development in fields such as laser beam printers for silver salt photosensitive materials, laser light sources for optical recording, and the like is anticipated. The beam, however, has a large diameter of several tens of μm. Therefore, there has been a problem with this method in that the obtained wavelength conversion efficiency is lower than that of a light wavelength conversion element of a light guiding wave channel type capable of producing beams with a narrow diameter.
Further, in order to generate a second harmonic wave, it is necessary to stabilize fluctuations of the wavelength of the fundamental wave emitted from a semiconductor laser to within 0.5 nm. If the fluctuation is not within the allowable wavelength range, the output of the second harmonic wave fluctuates due to the fluctuation of the wavelength of the semiconductor laser. An oscillation wavelength of a generally used excitation type semiconductor laser fluctuates by several nm due to changes in the driving current, changes in surrounding temperature, and even due to a very small return light. Therefore, there has been a problem with light wavelength conversion elements which utilize a semiconductor laser as a light source in that the output light amount thereof fluctuates when a second harmonic wave is generated, so that light cannot be output continuously.
Moreover, when the light wavelength conversion element comprising the above-mentioned periodic domain reversing structure is used, the wavelength conversion efficiency decreases significantly unless the oscillation wavelength of the semiconductor laser coincides with a wavelength which phase-matches with the period of a domain reversing segment. As a result, it is difficult to obtain a light source with a shorter wavelength which is of practical use.